JPH11231358A - Optical circuit and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the operation voltage in a waveguide type optical control device using electrooptic crystal. SOLUTION: In an optical waveguide consisting of electrooptic crystal, a selectively domain-inverted part 15 is provided. By this structure, only by applying electric fields in the same direction to respective optical waveguides, the refractive index changes of opposite polarities is induced in plural parts where refractive index changes should be caused, and the operation voltage can be reduced. For example, when this method is applied to a Mach-Zehnder phase shifter, what is called push-pull operation is possible by application of an electric field in one direction, and further, two-fold refractive index difference is given between two optical waveguides to reduce the operation voltage furthermore.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical circuit and other optical elements, and more particularly, to a waveguide type optical circuit and other optical elements which are controlled by using an optical waveguide provided in a substrate.

[0002]

2. Description of the Related Art In recent years, as optical communication systems have been put into practical use, higher capacity and higher functioning optical communication systems have been desired. For this purpose, higher speed optical modulators and optical control elements such as optical switches are required. It has become. To realize these functions, lithium niobate LiNbO ₃ (hereinafter, referred to as LiNbO ₃ )
"LN") and lithium tantalate LiTaO ₃ (
Hereinafter, referred to as “LT”).
A diffusion type optical waveguide in which titanium, magnesium, protons, or the like are doped from the substrate surface by thermal diffusion is mainly used.

[0003] With respect to a waveguide type optical circuit which performs control using these diffusion type optical waveguides, for example, Okayama et al. Refer to the IEICE Tech., TSSE94-214, and Nishimoto et al.
As described in Vol. 1, No. 8, page 521, many optical circuits can be integrated in one element by using ordinary photolithography technology. In addition, as an optical modulator utilizing the speed of response of these electro-optic crystals,
Mach-Zehnder type high-speed optical modulators have been put to practical use.
As an optical switch, a digital optical switch that switches output waveguides to which light is coupled by simultaneously applying voltages having different polarities to two to three Y-branch output waveguides has been proposed. .

As another method of forming an optical waveguide constituting these optical circuits, Japanese Patent Application Laid-Open Nos. 2-259608, 7-64034, 8-585-85 have been disclosed.
As proposed in Japanese Patent Application Publication No. 4 (1999) -1995, there is a method of forming a ridge-type optical waveguide by a hetero-epitaxial growth technique using a molten flux. Further, although not for the purpose of producing an optical circuit, a method of partially reversing the crystal orientation of LN (domain reversal) is disclosed in Japanese Patent Laid-Open No. 3-488.
No. 32, JP-A-4-270322, JP-A-5
Japanese Patent Application Laid-Open Nos. 220201, 5-172295 and 5-100271. These methods are for performing quasi-phase matching in the second harmonic generation element using an LN crystal.

Japanese Patent Application Laid-Open No. 7-5404 proposes an electro-optical modulator that compensates for phase speed mismatch between an optical signal and a high-frequency signal. This electro-optical modulator
An optical waveguide formed on the ferroelectric substrate and coupled to the optical input. The high frequency waveguide is formed on a substrate to apply an electric field to a region adjacent to the optical waveguide and modulate an optical signal. The ferroelectric substrate has a ferroelectric domain region for compensating for a phase difference in the modulation region, and the ferroelectric domain region is periodically inverted and non-inverted.

[0006]

In order to realize a practical optical modulator or optical switch using LN, it is important to lower the operating voltage as much as possible. In a digital optical switch, it is necessary to simultaneously apply a voltage of about ± 50 V to two output waveguides due to its structure, and it is effective to lower this voltage even a little. Also, in the Mach-Zehnder type optical switch, it is necessary to apply a high voltage of about 50 V to the two phase shifters, but it is effective to lower this voltage as in the case of the optical switch. In a high-speed optical modulator, a lower drive voltage leads to a driver circuit that can operate at a higher speed. Therefore, in the high-speed optical modulator, it is desired to lower the operating voltage even a little in order to drive the driver circuit at high speed.

SUMMARY OF THE INVENTION The present invention has been made in view of this point, and has as its object to reduce the operating voltage of an optical modulator, an optical switch, and other optical functional devices using the electro-optic effect.

[0008]

According to the present invention, there is provided an optical functional device having an optical waveguide made of an electro-optic crystal such as LN or LT. The basic structure is to apply an electric field in the same direction to each optical waveguide. The present invention can be applied to an optical circuit, an optical modulator, an optical switch, and the like as described below.

Specifically, as described in claim 1, the present invention provides an optical circuit comprising an optical waveguide made of an electro-optic crystal, wherein the optical waveguide is formed of a plurality of optical waveguides having different domain directions. An optical circuit is provided. According to a second aspect of the present invention, in the optical modulator having the optical waveguide made of the electro-optic crystal, the domain directions of the two optical waveguides forming the phase shifter are respectively reversed. An optical modulator is provided.

Furthermore, in the optical switch having two optical waveguides functioning as a phase shifter and made of an electro-optic crystal, the domain directions of the two optical waveguides are reversed. An optical switch is provided. Further, as described in claim 4, each of the plurality of output optical waveguides having an input optical waveguide and an output optical waveguide made of an electro-optic crystal and connected to one input optical waveguide in a Y-branch shape. By changing the refractive index by an electric field, in a digital optical switch having a function of selecting an output optical waveguide from which optical output is obtained among a plurality of output optical waveguides, the domain directions are inverted between the output waveguides, respectively. An optical switch is provided.

Furthermore, in a directional coupler type optical switch having an optical waveguide made of an electro-optic crystal, the domain direction is inverted between adjacent optical waveguides. An optical switch is provided. In the method of manufacturing the optical circuit, the optical modulator, and the optical switch, it is preferable that at least a part of them is formed by liquid phase epitaxy (LPE).

[0012]

By using the optical circuit, the optical modulator and the optical switch according to the present invention described above, it is possible to apply a unidirectional electric field to a plurality of portions where a change in the refractive index is required, and to obtain a refraction having the opposite polarity. A rate change can be induced. Therefore, for example, in a Mach-Zehnder type phase shifter, a so-called push-pull type operation can be performed by applying an electric field in one direction. By providing a double refractive index difference between the two optical waveguides, the operating voltage can be further reduced.

[0013]

Next, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a perspective view showing an optical waveguide and an electrode structure of a commonly used Mach-Zehnder type optical modulator (hereinafter, “Mach-Zehnder type” is simply referred to as “MZ type”). MZ type optical interference system 1 on LN substrate 10
1 is formed by the Ti diffusion method, and one signal electrode 12 is connected to the MZ type optical interference system 11. Further, the signal electrode 12 is surrounded so as to surround the signal electrode 12.
Ground electrodes 13a, 13b inside and outside
Are formed. When an electric field is applied to the MZ optical modulator, the intensity of the applied electric field is distributed almost only under the signal electrode 12.

FIG. 2 shows the operating characteristics of the MZ optical modulator normally obtained. FIG. 2 shows the optical output of the MZ optical modulator on the vertical axis,
This graph shows the applied voltage on the horizontal axis. As can be seen from this graph, a typical MZ-type optical modulator typically has:
An operating voltage of about 3.5 V is required. FIG. 3 shows an optical waveguide and an electrode structure of the MZ optical modulator according to the embodiment of the present invention. An MZ-type optical interference system 14 is formed on the LN substrate 10 by, for example, a Ti diffusion method, and the MZ-type optical interference system 14 has two signal electrodes 12 on one side of the MZ-type optical interference system 14. Side is connected to each. further,
Ground electrodes 13a and 13b are formed inside and outside the signal electrode 12 so as to surround the signal electrode 12, respectively. In the MZ type optical modulator according to the present embodiment, a domain inversion unit 15 is provided on one arm of the phase shifter in the MZ type optical interference system 14.

In order to drive the MZ-type optical modulator according to the present embodiment, voltages having the same phase are applied to the two signal electrodes 12. To give in-phase voltages,
For example, one electrode wiring may be branched into two outside the chip, or may be branched into two inside the chip. what if,
When the domain inversion unit 15 is not provided, it is necessary to apply voltages of opposite phases to the two signal electrodes 12. For that purpose, a special electric circuit is required. Furthermore, when performing high-speed driving, it is necessary to make fine adjustments so that the electric signals supplied to the two signal electrodes 12 have exactly the opposite phases. Therefore, it is extremely difficult to realize high-speed driving.

On the other hand, in the MZ type optical modulator according to the present embodiment, since the voltages applied to the two signal electrodes 12 are in the same phase, a special circuit for providing an electric circuit of the opposite phase is used. No electrical circuit is required. According to the MZ type optical modulator according to the present embodiment, the operating voltage is reduced to about half as compared with the ordinary optical intensity modulator as shown in FIG. FIG. 4 shows typical operating characteristics of the MZ optical modulator according to the present embodiment. The operating voltage is about 1.9V, which is
The operating voltage of the conventional MZ type optical modulator shown in FIG.
Is almost half. As described above, the MZ optical modulator according to the present embodiment has an effect of lowering the operating voltage of the optical modulator without using a special electric circuit.

The MZ type optical interference system 14 of the MZ type optical modulator according to the present embodiment is composed of a buried optical waveguide or a ridge type waveguide formed using liquid phase epitaxy (LPE). It can also be made using a normal Ti diffusion waveguide. Next, FIG. 5 shows a method of manufacturing the domain inversion section 15 in the case of using a buried optical waveguide formed by using liquid phase growth.

First, as shown in FIG. 5A, LN, M
g-doped LN, Li (Nb _x Ta _{1 -x} ) O ₃ (0 ≦ X ≦
0.5) A first groove 52 for forming a domain inversion portion is formed in a substrate 51 made of any of the materials described above. For forming the first groove 52, selective wet etching or the like using ion beam etching or proton exchange can be applied.

Next, as shown in FIG.
The core material 53 whose domain direction is reversed from that of the
1 is formed so as to cover the surface. Next, as shown in FIG. 5C, the core material 53 is polished so that the core material 53 generally remains only in the first groove 52. Thereby, a domain inversion portion is formed.

Thereafter, as shown in FIG. 5D, a second groove 54 for forming a domain normal rotation portion is formed in the first groove 5.
As in the case of 2, it is formed by ion beam etching or the like. Next, as shown in FIG.
A core material 55 equal to the first material is formed so as to cover the surface of the substrate 51. Next, as shown in FIG. 5F, the core material 55 is polished so that the core material 55 remains substantially only in the second groove 54. In this way, the domain normal rotation section is completed.

The domain direction of the films 53 and 55 to be formed is controlled by a difference in the coefficient of thermal expansion between the material of the substrate 51 and the film forming material due to a difference in composition.
It is performed using the principle that crystal growth occurs in an orientation where the lattice constants are more consistent. Therefore, by adjusting the crucible temperature during crystal growth to an appropriate temperature, the domain directions of the films 53 and 55 can be arbitrarily adjusted.

The method shown in FIG. 5 is only one example of forming the domain inversion section 15. For example, there is substantially no problem even if the order of forming the domain inversion portion and the domain normal rotation portion is reversed. Also, as shown in FIG.
In addition to the method of digging and embedding the core materials 53 and 55, for example, after forming a film having a predetermined composition and crystal orientation,
The film may be etched to form a channel waveguide.

As another method of forming the domain inversion portion, there is a method of selectively performing domain inversion after forming a waveguide having a certain domain direction by Ti diffusion or the like. For example, a method of performing domain inversion by applying a high voltage to only a predetermined portion of a waveguide while maintaining an appropriate high temperature state, or causing domain inversion after selectively making domain inversion easily by proton exchange or the like Method and the like.

FIG. 6 is a perspective view showing another example of the electrode structure in the optical modulator according to the present embodiment. LN substrate 1
An MZ type optical interference system 14 is formed on 0 by, for example, a Ti diffusion method, and one signal electrode 12 is connected to the MZ type optical interference system 14. Further, the signal electrode 1
Ground electrodes 13a and 13b are formed on the inside and outside of the signal electrode 12, respectively, so as to surround the signal electrode 2. A domain inversion section 15 is provided on one arm of the phase shifter in the MZ type optical interference system 14.

In this optical modulator, a signal electrode 12 is provided at the center of a phase shifter, and a ground electrode 1 is provided.
3 has a structure in which the end portion partially covers the waveguide. Even when this structure is used, the operation of the optical modulator becomes a so-called push-pull type due to the presence of the domain inversion unit operation 15, and the operating voltage can be reduced. FIG.
FIG. 9 is a perspective view showing still another example of the electrode structure in the optical modulator according to the embodiment.

The optical modulator shown in FIG. 7 has the same structure as the optical modulator shown in FIG. That is, the LN substrate 1
0, an MZ type optical interference system 14 is formed.
One signal electrode 12 is connected to the Z-type optical interference system 14. Furthermore, ground electrodes 13 are respectively provided inside and outside the signal electrode 12 so as to surround the signal electrode 12.
a and 13b are formed. A domain inversion section 15 is provided on one arm of the phase shifter in the MZ type optical interference system 14.

In this optical modulator, the signal electrode 12 is provided so as to cover the two phase shifters through the center of the phase shifter, and the ground electrode 13 is provided close to the signal electrode 12. ing. Even if this structure is used, the operation of the optical modulator becomes a so-called push-pull type due to the presence of the domain inversion unit 15, and the operating voltage can be reduced.

Although the electrode structure of the optical modulator shown in FIG. 7 is almost similar to that of the optical modulator shown in FIG. 6, when designing the optical modulator shown in FIG. The optimum structure can be selected by appropriately selecting the spacing between the phase shifters and other waveguide layout design parameters and the ratio of (electrode width) to (electrode gap) and other electrode design parameters.

Although the above example is an example in which the present invention is applied to an optical intensity modulator, the present invention can be applied to an optical switch. For example, by deflecting the Y-branch of the MZ optical modulator to a 3 dB coupler composed of a directional coupler, the operating voltage of the MZ optical switch can be reduced with the same electrode configuration as that shown in FIGS. It is possible to reduce.

As one mode of the optical switch, there is a digital optical switch based on a Y-branch. FIG. 8 shows a typical waveguide and electrode configuration. A pair of signal electrodes 12 is provided so as to surround a branch point of the waveguide branched into a Y-shape, and one of the signal electrodes 12 is connected to the ground electrode 13. In this digital optical switch, by applying a voltage of about ± 50 V to the signal electrode 12 with the ground electrode 13 as a reference,
An output port to which light is coupled can be selected.

FIG. 9 is a plan view showing the structure of an optical switch according to one embodiment of the present invention. In the optical switch according to the present embodiment, the first signal electrode 12a is provided inside the branch point of the waveguide branched into a Y-shape, and the second signal electrodes 12a are provided on both sides of the Y-shape branch point, respectively. 12b is provided. The second signal electrodes 12 b on both sides of the Y-shaped branch point are all connected to the ground electrode 13. A domain inversion unit 15 is provided on one arm of the waveguide branched into a Y-shape.
Is provided.

According to the Y-branch type optical switch according to the present embodiment, the domain inversion section 15 is provided on one arm of the waveguide, so that an electric field in the same direction is applied to the two output waveguides. A change in the refractive index in the opposite direction can be induced in each of the two output waveguides, and the operating voltage can be further reduced. FIG. 10 is a plan view showing a waveguide structure and an electrode layout in a normal optical switch using a directional coupler. Two waveguides are provided in parallel with each other, and a signal electrode 12 is provided on each of the waveguides in a region where the distance between the two waveguides is partially reduced. One of the signal electrodes 12 is connected to the ground electrode 13.

FIG. 11 is a plan view showing a waveguide structure and an electrode layout of an optical switch according to an embodiment of the present invention. According to the present embodiment, in a region where the interval between these two waveguides is partially narrow, the first signal electrode 12a is provided outside each of the waveguides. The second signal electrode 12b is provided so as to straddle both of the wave paths. The pair of first signal electrodes 12a are both connected to the ground electrode 13. In a region where the interval between the two waveguides is partially narrow, one of the waveguides is provided with a domain inversion section 15.

According to the optical switch of this embodiment, the domain inversion section 15 is provided in one of the waveguides.
By applying an electric field in the same direction to the two output waveguides, a change in the refractive index in the opposite direction can be induced in each of the two output waveguides, and the operating voltage can be further reduced.

[0035]

As described above, according to the present invention, lithium niobate LiNbO ₃ and lithium tantalate LiTa are used.
In an optical waveguide made of an electro-optic crystal such as O ₃ , a portion where an electrode is applied is selectively domain-inverted. In such a partially domain-inverted optical waveguide, it is possible to induce a refractive index change of opposite polarity by simply applying an electric field in the same direction to a plurality of portions where a refractive index change is to occur. And the operating voltage can be reduced.

Therefore, when the present invention is applied to, for example, a Mach-Zehnder type phase shifter, a so-called push-pull type operation becomes possible only by applying an electric field in one direction.
By providing a refractive index difference between the two optical waveguides, the operating voltage can be further reduced.

[Brief description of the drawings]

FIG. 1 is a perspective view showing an optical waveguide and an electrode structure of a general Mach-Zehnder type light intensity modulator.

FIG. 2 is a graph showing operation characteristics of the Mach-Zehnder type optical intensity modulator shown in FIG.

FIG. 3 is a perspective view showing an optical waveguide and an electrode structure of a Mach-Zehnder type optical intensity modulator according to one embodiment of the present invention.

FIG. 4 is a graph showing operation characteristics of the Mach-Zehnder type optical intensity modulator shown in FIG.

FIGS. 5A to 5F are perspective views showing a process of forming a domain inversion portion in the Mach-Zehnder type light intensity modulator according to one embodiment of the present invention.

FIG. 6 is a perspective view showing an optical waveguide and an electrode structure of a Mach-Zehnder type optical intensity modulator according to another embodiment of the present invention.

FIG. 7 is a perspective view showing an optical waveguide and an electrode structure of a Mach-Zehnder type optical intensity modulator according to still another embodiment of the present invention.

FIG. 8 is a plan view showing an optical waveguide and electrode structure of a typical Y-branch digital optical switch.

FIG. 9 is a plan view showing an optical waveguide and an electrode structure of a Y-branch digital optical switch according to an embodiment of the present invention.

FIG. 10 is a plan view showing an optical waveguide and electrode structure of a typical directional coupler type optical switch.

FIG. 11 is a plan view showing an optical waveguide and an electrode structure of a directional coupler type optical switch according to an embodiment of the present invention.

DESCRIPTION OF SYMBOLS 10 LN substrate 11 Mach-Zehnder type optical interference system 12 signal electrode 12 a first signal electrode 12 b second signal electrode 13 ground electrode 14 MZ type optical interference system 15 domain inversion section 51 substrate 52 for forming domain inversion section First groove 53 Core material with domain inversion 54 Second groove 55 for forming domain normal rotation part 55 Core material

Claims (10)

[Claims] 1. An optical circuit comprising an optical waveguide made of an electro-optic crystal, wherein the optical waveguide comprises a plurality of optical waveguides having different domain directions. 2. An optical modulator having an optical waveguide made of an electro-optic crystal, wherein the domain directions of two optical waveguides forming a phase shifter are respectively inverted. 3. An optical switch comprising an electro-optic crystal and having two optical waveguides functioning as a phase shifter, wherein the domain directions of the two optical waveguides are respectively reversed. 4. An optical waveguide comprising an input optical waveguide and an output optical waveguide made of an electro-optic crystal, wherein the refractive indexes of a plurality of output optical waveguides connected to one input optical waveguide in a Y-branch are changed by an electric field. Accordingly, in a digital optical switch having a function of selecting an output optical waveguide from which a light output can be obtained among a plurality of output optical waveguides, the domain direction is inverted between the output waveguides. Switch. 5. A directional coupler type optical switch having an optical waveguide made of an electro-optic crystal, wherein the domain direction is inverted between adjacent optical waveguides. 6. A method for manufacturing an optical circuit comprising an electro-optic crystal and comprising a plurality of optical waveguides having different domain directions, wherein at least a part of the optical circuit is formed by liquid phase growth. Manufacturing method of an optical circuit. 7. A method for manufacturing an optical modulator having an optical waveguide made of an electro-optic crystal, wherein the domain directions of two optical waveguides forming a phase shifter are respectively inverted. A method for manufacturing an optical modulator, wherein at least a part of the device is formed by liquid phase growth. 8. An optical switch made of an electro-optic crystal and having two optical waveguides functioning as a phase shifter,
A method for manufacturing an optical switch in which the domain directions of the two optical waveguides are respectively inverted, wherein at least a part of the optical switch is formed by liquid phase growth. 9. An optical waveguide comprising an input optical waveguide and an output optical waveguide comprising an electro-optic crystal, wherein the refractive indexes of a plurality of output optical waveguides connected to one input optical waveguide in a Y-branch are changed by an electric field. Accordingly, a digital optical switch having a function of selecting an output optical waveguide from which a light output can be obtained among a plurality of output optical waveguides, wherein an optical switch whose domain direction is inverted between the output waveguides is manufactured. A method of manufacturing an optical switch, wherein at least a portion of the optical switch is formed by liquid phase growth. 10. A method for manufacturing a directional coupler type optical switch having an optical waveguide made of an electro-optic crystal, wherein a domain direction is inverted between adjacent optical waveguides. Characterized in that at least a part of the optical switch is formed by liquid phase growth.